Scheduling and queue management with adaptive queue latency

ABSTRACT

The invention relates to a scheduler for a TCP/IP based data communication system and a method for the scheduler. The communication system comprises a TCP/IP transmitter and a receiving unit (UE). The scheduler is associated with a Node comprising a rate measuring device for measuring a TCP/IP data rate from the TCP/IP transmitter and a queue buffer device for buffering data segments from the TCP/IP transmitter. The scheduler is arranged to receive information from the rate measuring device regarding the TCP/IP data rate and is arranged to adapt the permitted queue latency to a minimum value when the TCP/IP transmitter is in a slow start mode and to increase the permitted queue latency when the TCP/IP rate has reached a threshold value.

TECHNICAL FIELD

The invention relates to a scheduler for a TCP/IP based datacommunication system comprising a TCP/IP transmitter and a receivingunit, UE. The scheduler is associated with a Node comprising a ratemeasuring device for measuring a TCP/IP data rate from the TCP/IPtransmitter and a queue buffer device for buffering data segments fromthe TCP/IP transmitter. The scheduler is arranged to receive informationfrom the rate measuring device regarding the TCP/IP data rate and thescheduler is arranged to communicate with the buffer device for activequeue management AQM. The invention also relates to a method for such ascheduler.

BACKGROUND

The Transmission Control Protocol (TCP) is a virtual circuit protocolthat is one of the core protocols of the Internet protocol suite, oftensimply referred to as TCP/IP. Using TCP, applications on networked hostscan create connections to one another, over which they can exchangestreams of data. The protocol guarantees reliable and in-order deliveryof data from sender to receiver. TCP also distinguishes data formultiple connections by concurrent applications (e.g. Web server ande-mail server) running on the same host.

TCP supports many of the Internet's most popular application protocolsand resulting applications, including the World Wide Web, e-mail andSecure Shell.

In the Internet protocol suite, TCP is the intermediate layer betweenthe Internet Protocol (IP) below it, and an application above it.Applications often need reliable pipe-like connections to each other,whereas the Internet Protocol does not provide such streams, but ratheronly best effort delivery (i.e. unreliable packets). TCP does the taskof the transport layer in the simplified OSI model of computer networks.The other main transport-level Internet protocol is UDP.

Hence, TCP/IP is a protocol used for internet traffic for devicescomprising a computer. This protocol allows for transmitting andreceiving a data stream comprising a number of data segments in the formof packages. The data stream is advantageously broken up so that thesegments may be sent to and from a user equipment (hereinafter calledUE) with as high data transmission rate (hereinafter called the rate) aspossible, i.e. with as many transmitted segments per time unit aspossible. The UE intended to receive the data segments comprises areceiver for receiving the segments and for assembling the segments intoa data stream corresponding to the transmitted data stream. When the UEhas received a segment an acknowledgement signal, ACK, is sent back tothe UE that transmitted the segment. If a segment is not acknowledgedthe transmitter retransmits the segment. The TCP/IP protocol includesrate control in order to avoid congestion in the network. Two differentrate control phases (or control modes) exist and they are “slow start”and “congestion avoidance”. A transmission always starts in slow startmode with a low rate. Then for each roundtrip time, RTT, the rate isdoubled. The TCP/IP transmitter also estimates the RTT based on the timewhen a message is transmitted until corresponding acknowledge message isreceived from the TCP/IP receiver.

Two different reasons result in that the TCP/IP transmitter reduces itsrate:

1) The transmitter does not receive an ACK on a transmitted segmentwithin a roundtrip time timeout, RTO. The RTO is determined by afunction based on the estimated RTT. When this RTO has expired, thetransmitter restarts with the slow start mode.2) A duplicated ACK is received a number of times, often three. Aduplicate ACK is an ACK that point out the same segment number as theprevious ACK pointed out. This means that the transmitter knows that thereceiver did not receive the segment appointed to, in the ACK.Furthermore, the transmitter also knows that the receiver has receivedsome later transmitted segments. Hence, the transmitter knows that theconnection is still in operation but that a segment is lost. In such acase the transmitter performs a fast retransmission and reduces itstransmission rate to half the current transmission rate. This handlingis included in order to avoid that the transmitter restarts with thetime consuming slow start mode, which is the consequence if an ACK isnot received by the transmitter within the RTO i.e. if the missingsegment is not retransmitted and acknowledged within the RTO.

In mobile networks, (e.g. IEEE 802.16 and 3rd Generation PartnershipProject (3GPP)) a scheduler exists in order to distribute thetransmission capacity fairly between different data transmission flowsto different UEs in a cell over an air interface. At the same time thescheduler tries to utilize the transmission capacity in such a way sothat the total transmission rate in the network, i.e. e.g. in the cell,can be maximized. Since the transmission capacity is limited in the airinterface, the scheduler wants to control the rate of each transmissionflow in a fair manner.

Several methods exist in order to regulate the transmission flow rate.One method is to incorporate a control-signaling interface between themobile network and the TCP/IP transmitter side, where the networkexplicitly informs the transmitter what rate is acceptable for thescheduler.

Another method is that the mobile network provokes the TCP/IP protocolto reduce its rate, when needed, by discarding one segment buttransmitting the following segments further to the receiver in the UE.This will result in a lost ACK and that the TCP/IP transmitteridentifies that a segment is lost and that the transmitter will go intoa fast retransmission mode according to item 1 above, i.e. to retransmitthe lost segment and to reduce the TCP/IP rate to half the currenttransmission rate. This method strives to force the TCP/IP to a fastretransmission occasion instead of an RTO occasion that leads to thetime-consuming slow start mode. Hence, the scheduler comprised in themobile network can control the TCP/IP rate so that the scheduling ratecontrol and the TCP/IP rate control interact in a good manner. This ideais adopted in a function often called Active Queue Management, AQM.

Furthermore, to establish a fast TCP/IP rate control, in order to get ahigh average transmission rate, it is known to minimize the round triptime. In order to minimize the RTT it is desired to minimize datalatency time in the mobile network since the data latency is oneparameter affecting the RTT. However, the TCP/IP protocol adjusts itsrate for each received ACK. Hence, the faster an ACK is received, thefaster the TCP/IP rate can increase, and the shorter RTT and latencytime is necessary. This is a problem since channel performance between abase station and the UE always varies over time, especially when the UEis moving. When several UEs are connected to the cell, the channelperformance is unique for each UE connection. Hence good channelperformance will arise at different times for the different connections.

If then, according to the above, the permitted latency is minimized inthe network in order to get a fast TCP/IP rate control the scheduler hasto transmit data to an UE irrespective of whether if the momentarychannel performance is good or bad. This means that the data may betransmitted on a poor channel requiring a lot of transmission capacitye.g. transmission power, and maybe a lot of retransmissions. This willhave a negative impact on the scheduling gain for the network, i.e. forthe total transmission rate in the network, i.e. e.g. in the cell. Theproblem also increases with the number of UEs in the cell since the moreUEs being scheduled on bad channels; the more transmission capacity isrequired.

For the reasons above there exists a need for an improved scheduler thatcan manage the balance between TCP/IP rate using AQM and good channelperformance for each UE in a cell controlled by the scheduler so thatincreased scheduling gain is achieved for the cell.

SUMMARY

The object of the invention is to meet the above need with a schedulerthat can handle a TCP/IP data communication system so that schedulinggain is increased in a system comprising a number of user equipments.Here scheduling gain refers to the situation where a scheduler in aTCP/IP based system can control and time a number of devices so that aTCP/IP transmitter can increase its transmission rate of TCP/IP baseddata segments.

Hence, the invention refers to a method for scheduling a TCP/IP basedsystem and a scheduler for controlling the system. The TCP/IP basedsystem comprises the TCP/IP transmitter and a receiving unit. The TCP/IPtransmitter may be comprised in a user equipment, UE, during upload ormay be comprised in a transceiver unit in a part of a wireless networkduring download. Here “upload” and “download” is with reference to theUE, i.e. when the UE sends information the UE performs an upload andwhen the UE receives information the UE performs a download. The Nodemay be a base transceiver station comprising a transmitter and areceiver.

The Node comprises a rate measuring device for measuring a TCP/IP datarate from the TCP/IP transmitter and a queue buffer device for bufferingdata segments from the TCP/IP transmitter. The scheduler is arranged toreceive information from the rate measuring device regarding the TCP/IPdata rate, i.e. the number of data segments being transmitted per timeunit. The scheduler is arranged to communicate with the buffer devicefor active queue management, AQM, according to prior art.

The invention is characterised in that the scheduler is arranged toperform the following steps:

-   -   adapt the permitted queue latency to a minimum value when the        TCP/IP transmitter is in a slow start mode and to;    -   increase the permitted queue latency when the TCP/IP rate has        reached a threshold value.

One advantage of the invention is that the adaptive increase of thepermitted queue latency will allow for the scheduler to allocate thetransmission of the segments buffered in the queue buffer device to atime period where the channel quality is good. Hence, there will be lessuse of transmission capacity, e.g. less lost segments and lessretransmission and thus fewer periods where the TCP/IP transmitter isforced into the slow start mode, why the scheduling gain is increased.

One further advantage of the invention is that the permitted queuelatency can be set to be very low during the slow start mode which willshorten the time period for the TCP/IP rate to reach a maximum value. Inother words, the increase in the TCP/IP rate per time period can beincreased by use of the short permitted queue latency.

The scheduler is arranged to increase the permitted queue latency untilit reaches a selected maximum permitted latency time. This selectedmaximum permitted latency time depends on a number of factors, forexample, the RTO and the number of UEs. However, the scheduler isarranged to increase the permitted queue latency with a rate low enoughto avoid a round trip time out, RTO.

The scheduler is advantageously arranged to increase the permitted queuelatency when, or after, the threshold value for the TCP/IP rate isreached. This threshold value corresponds to when the scheduler uses theAQM to force the TCP/IP transmitter to a fast retransmission mode. Here,the scheduler is arranged to use the AQM to control the queue buffer todrop at least one data segment, preferably the oldest segment, when theTCP/IP rate has reached its threshold value, in order to force theTCP/IP transmitter into a fast retransmission mode instead of risking aslow start mode.

The benefit of this embodiment is that the TCP/IP rate can be kept at ahigh level and that the scheduler continuously monitors the TCP/IP rateso that the permitted queue latency can be increased when the TCP/IPrate is high. The fast retransmission mode is an indicator of a highTCP/IP rate which is why this is a suitable trigger for increasing thepermitted queue latency.

Furthermore, the monitoring of the TCP/IP rate allows for the schedulerto be arranged to reset the permitted queue latency to its minimum valueif the measured TCP/IP rate is below a selected minimum rate in order toidentify if the TCP/IP is in a slow start mode.

In one embodiment of the invention, the scheduler is advantageouslyarranged to increase the permitted queue latency stepwise when, orafter, the scheduler uses the AQM to force the TCP/IP transmitter to thefast retransmission mode. However, the permitted queue latency may beincreased continuously.

As mentioned above the data communication system may be a wirelessnetwork, for example, a telephone and/or data communication network,comprising an air interface between the TCP/IP transmitter and thereceiving unit.

BRIEF DESCRIPTION OF DRAWINGS

The invention will below be described in connection to a number ofdrawings in which;

FIG. 1 schematically shows a flow chart for downlink in a TCP/IP basednetwork comprising a scheduler according to the invention;

FIG. 2 schematically shows a flow chart for uplink in a TCP/IP basednetwork comprising a scheduler according to the invention;

FIG. 3 a schematically shows a diagram of TCP/IP rate dependent on timeaccording to prior art for one pair of TCP/IP transmitter and UE;

FIG. 3 b schematically shows a diagram of permitted queue latencydependent on time according to prior art for one pair of TCP/IPtransmitter and UE;

FIG. 4 a schematically shows a diagram of TCP/IP rate dependent on timeaccording to the present invention for one pair of TCP/IP transmitterand UE;

FIG. 4 b schematically shows a diagram of permitted queue latencydependent on time according to the present invention for one pair ofTCP/IP transmitter and UE;

FIG. 5 a schematically shows a diagram of channel quality dependent ontime for a UE according to prior art;

FIG. 5 b schematically shows a diagram of transmitted data from a TCP/IPtransmitter dependent on time according to prior art;

FIG. 5 c schematically shows a diagram of selected transmissionopportunities when the maximum permitted queue latency is shortaccording to prior art;

FIG. 6 a is the same as FIG. 5 a;

FIG. 6 b is the same as FIG. 5 b, and in which;

FIG. 6 c schematically shows a diagram of selected transmissionopportunities when the maximum permitted queue latency is long accordingto the present invention.

DETAILED DESCRIPTION

FIG. 1 schematically shows a flow chart for downlink in a TCP/IP basednetwork comprising a scheduler according to the invention. In FIG. 1 aTCP/IP based system comprises a transceiver unit comprising a TCP/IPtransmitter and a receiving unit in the form of a user equipment, UE,comprising a TCP/IP receiver and a Node there between.

The UE is a mobile unit in a wireless network that communicates with thetransceiver unit via the Node. The UE communicates with the Node via anair interface, i.e. a wireless communication link. In GSM (Global Systemfor Mobile Communication) and UMTS (Universal Mobile TelecommunicationsSystem) the air interface is called a radio access network and is theradio frequency portion of the circuit between the UE and the Node.

It should be noted that the transceiver unit can be a second UE or a UEfixed in a wired network or a server in a wired network or any equipmentusing TCP/IP for end-to-end communication, i.e. transmission andreception, of data to and from the UE. The Node can, for example, be abase transceiver station in a GSM system or a Node B in a WCDMA(Wideband Code Division Multiple Access) based system.

In a base transceiver station, latency refers to the amount of time ittakes for a packet to travel from the input port to the output port ofthe base station. The latency in the base station contributes to thetotal round trip time, RTT, that a TCP/IP session observes.

The queue buffer stores a number of data packets, i.e. a number of datasegments, and forwards the packets to the BTS transmitter dependent onthe scheduler that controls the queue buffer. The BTS transmitter thenforwards the packets to the actual UE.

The scheduler according to the invention is associated with the Nodecomprising a rate measuring device for measuring a TCP/IP data rate fromthe TCP/IP transmitter and a queue buffer device for buffering datasegments from the TCP/IP transmitter. Here “associated” refers to thescheduler being comprised in the Node or that the scheduler is anexternal unit being connectable to the Node. The data transmission ratecomprises information on momentary data rate. The rate measuring deviceforwards information about the data transmission rate to the scheduler.

In FIG. 1 the Node also comprises a Node transmitter for transmittingdata segments from the queue buffer device to the UE and a Node receiverfor receiving data segments from the UE. In FIG. 1 the UE comprises aTCP/IP transmitter for transmitting data segments to the Node and thetransceiver unit comprises a TCP/IP receiver for receiving the datasegments from the Node.

As mentioned above, the scheduler is arranged to receive informationfrom the rate measuring device regarding the TCP/IP data rate. Thescheduler is also arranged to communicate with the buffer device foractive queue management AQM. Hence, the scheduler comprises AQM. Here,the scheduler is arranged to use the AQM to control the queue buffer todrop at least one data segment, preferably the oldest segment, when theTCP/IP rate has reached its threshold value, in order to force theTCP/IP transmitter into a fast retransmission mode instead of risking aslow start mode.

In FIG. 1 it is shown that the data communication system comprises anumber of transceiver units and a corresponding number of UEs. The Nodecomprises a corresponding number of rate measuring devices, queue bufferdevices and Node receivers. However, the Node comprises only onescheduler for scheduling all the above devices and one Node transmitterfor sending the data segments to the UE. The TCP/IP communication systemis an end-to-end system with one transceiver unit on one end and an UEat the other end. The data stream transmitted from the transceiver unitis thus dedicated to a certain UE at the other end and vice versa. Thescheduler chooses a number of UEs to share the total transmissioncapacity and the scheduler then allows a number of transmitters totransmit to a number of receivers chosen by the scheduler. Thetransmitter may be positioned in the transceiver unit or in the UE.However, in the Node there may be only one queue per UE or severalqueues per UE. Consequently, in the Node at least one rate measuringdevice and at least one queue buffer device are thus arranged to handlethe data communication between the dedicated transceiver unit and thededicated UE. When there is more than one pair of transceiver unit andUE at least a corresponding number of rate measuring devices, queuebuffers and receivers are used.

In one embodiment of the invention the data stream from the transceiverunit is not only a single data stream but may be a multiplexed signalcomprising a number of data streams, dedicated for one user, having beenput together into one data stream queue. The rate measuring device thenmeasures on the data stream queue instead of on the individual datastreams. This is a problem for the scheduler since it is the individualdata streams that have to be acknowledged by the UE in order for the UEto use the information in the individual data streams. Hence, in thisembodiment the scheduler may use the AQM to force the TCP/IP transmitterinto a fast retransmission mode not by dropping a dedicated segment fromthe buffer, but by dropping a segment chosen based on a statisticground. By “statistic ground” is meant that one segment is lost at acertain point in time and a second segment at another point in time andso on, so that each individual data stream in the data stream queue hasdropped a segment so that the TCP/IP transmitter is forced into a fastretransmission for all individual data streams. Another way to realisethis is to de-multiplex the data stream queue in the Node before therate measuring device so that the there is one queue buffer device foreach data stream. In this case the scheduler can drop a dedicatedsegment in each queue buffer according to above.

In FIG. 1 it is shown that the Node comprises only one Node transmitterarranged to transmit into a certain geographical area called a cell. TheNode transmitter is arranged to transmit to one or more UEs in the cellduring a scheduling interval, i.e. during that time period decided bythe scheduler where a certain number UEs and transceiver units areallowed to transmit.

In the case where there are several UEs in the cell the scheduler usesinformation about all the UEs for deciding to which UE the transmittershall transmit data segments stored in the corresponding buffer device.The scheduler uses a lot of parameters for deciding which UE is next inline. The parameters comprise information that can be used by thescheduler for fairness, i.e. an algorithm that controls that all UEs arebeing equally treated. However, one parameter comprises information onchannel quality and it should be noted that the channel qualityfluctuates over time (see FIGS. 5 a-c and 6 a-c). It is advantageous ifthe Node transmitter can transmit to the UE with the best channelquality in order to minimise the need of transmission capacity for theUE. According to the invention the scheduler may adapt the permittedbuffer queue latency dependent on the current situation. When the TCP/IPrate is high the scheduler controls the buffer device to increase itspermitted queue latency so that the scheduler can allocate transmissionsto the UEs during those time periods where the UEs have good channelquality. Here “allocate” means that the scheduler may use a longer timeinterval within which the data segments may be transmitted. Hence, thetransmission may be postponed within the time period, i.e. the latency,until the channel quality is good enough.

Furthermore, if the TCP/IP transmitter goes into a slow start mode thescheduler according to the invention is arranged to adapt the permittedqueue latency in the buffer device to a minimum value. The schedulerthen increases the permitted queue latency when the TCP/IP rate hasreached a threshold value.

It is thus obvious that the more UEs that are scheduled with thescheduler according to the invention, the higher the scheduling gain.

FIG. 2 schematically shows a flow chart for uplink in a TCP/IP basednetwork comprising a scheduler according to the invention. FIG. 2 isessentially identical to FIG. 1 but with the exception that the UEcomprises a TCP/IP transmitter for transmitting data to the transceivervia the Node. The transceiver unit is the receiving part and thereforecomprises a TCP/IP receiver. Hence, in the uplink case in FIG. 2 thedirection of the data stream is opposite from what is depicted in FIG.1.

In FIG. 2 the scheduler forwards scheduling information to the UEs inorder to decide which UEs that shall be permitted to transmit data. Asin FIG. 1 the scheduler controls the queue buffer device to drop theoldest received segment, or any other suitable segment, that have notyet been forwarded to the TCP/IP receiver when the scheduler hasdetected that the TCP-IP rate is too high. As in FIG. 1 the ratemeasuring device measures the transmitted data rata from the TCP/IPtransmitter (here in the UE), and forwards the information to thescheduler so that the scheduler can decide whether the TCP/IP rate hasreached its upper threshold value. The permitted queue latency handlingis equivalent to the downlink case in FIG. 1.

FIG. 3 a schematically shows a diagram of TCP/IP rate dependent on timeaccording to prior art for one pair of TCP/IP transmitter and UEaccording to prior art. In FIG. 3 a the diagram has been divided intofour time zones I-IV, by introduction of four dashed lines, in order tofacilitate the description of the diagram. In the first zone I, theTCP/IP transmitter is in a slow start mode and the TCP/IP rate increasesnon-linearly from a TCP/IP value of X_min until a maximum value X_max.The TCP/IP transmitter may be in a slow start mode for several reasons,for example if the UE roams into the cell or re-connects after beingshut off or after a round trip time out, RTO, or if the transceiver unithas been shut off or has lost contact for another reason. The reason forbeing in a slow start mode is that a TCP/IP standard is already knownfrom prior art and it should be noted that the above list is notexhaustive.

When the TCP/IP rate is equal to X_max the scheduler uses the AQM toforce the TCP/IP transmitter into a fast retransmission mode with aTCP/IP rate being half the rate of the current rate at the time when thescheduler forced the TCP/IP transmitter into the fast retransmissionmode. In FIG. 3 a the fast retransmission rate can be detected by thedrop in TCP/IP rate in the boundary between the first zone I and thesecond zone II. In the second, third and fourth Zone II-IV the TCP/IPincreases from the half the rate to X_max until the scheduler forces theTCP/IP transmitter into a fast retransmission.

In FIG. 3 a is depicted a long term mean TCP/IP rate being of the valueX1. The low permitted queue latency in FIG. 3 b results in a fast TCP/IPrate ramp up but the long term average data X_1 rate in FIG. 3 a isquite low since the scheduling gain is limited. This low long termaverage data rate is a result of the behaviour described in FIGS. 5 a,5b and 5 c below.

FIG. 3 b schematically shows a diagram of permitted queue latencydependent on time according to prior art. In FIG. 3 b it is shown thatthe permitted queue latency is constant and independent of the differentmodes described in FIG. 3 a.

FIG. 4 a schematically shows a diagram of TCP/IP rate dependent on timeaccording to the invention for one pair of TCP/IP transmitter and UE. InFIG. 4 a the diagram has been divided into four time zones I-IV, byintroduction of four dashed lines, in order to facilitate thedescription of the diagram. The four zones I-IV in FIG. 4 a correspondsto the four zones in FIG. 3 a. In the first zone I the TCP/IPtransmitter is in a slow start mode and the TCP/IP rate increasesnon-linearly from a TCP/IP value of X_min until a maximum value X_max.The TCP/IP transmitter may be in a slow start mode because of thereasons described in connection to FIG. 3 a.

When the TCP/IP rate is equal to X_max the scheduler uses the AQM toforce the TCP/IP transmitter into a fast retransmission mode with aTCP/IP rate being half the rate of the current rate at the time when thescheduler forced the TCP/IP transmitter into the fast retransmissionmode. In FIG. 4 a the fast retransmission rate can be detected by thedrop in TCP/IP rate in the boundary between the first zone I and thesecond zone II. In the second, third and fourth zone II-IV the TCP/IPincreases from half the rate to X_max until the scheduler forces theTCP/IP transmitter into a fast retransmission.

In FIG. 4 a is depicted a mean TCP/IP rate being of the value X2. Thevalue X2 is higher than the value X1 in FIG. 1 because of the schedulinggain due to the present invention where the permitted queue latency isadaptive. Since the permitted queue latency is adaptive the TCP/IPmaximum rate X_max may be higher than in prior art. The invention willbe further explained in connection to FIG. 4 b.

FIG. 4 b schematically shows a diagram of permitted queue latencydependent on time according to the invention. FIG. 4 b shows that thescheduler may control the buffer device so that the permitted queuelatency is adaptive with regard to the TCP/IP rate and the mode of theTCP/IP transmitter. In FIG. 4 b the permitted queue latency can be setto a value being LATENCY_MIN when the TCP/IP transmitter is in the slowstart mode in zone I. The LATENCY_MIN may be set to a lower value thanthe constant permitted queue latency in FIG. 3 b since the presentinvention allows for the permitted queue latency to be increased. Inprior art the constant permitted queue latency has to be set by to avalue being dependent on a trade off between being too long during theslow start mode and too short when the TCP/IP is up and running in itsnormal pace. When the TCP/IP transmitter is in a slow start mode it isadvantageous to use as short permitted queue latency as possible becausethe shorter the latency the higher the increase in TCP/IP rate. However,when the latency is short it consumes a lot of transmission capacity.However, when the TCP/IP has reached its maximum value X_max the need ofshort RTT is gone and the permitted queue latency can be increased.

For this reason the inventive scheduler increases the permitted queuelatency when the TCP/IP transmitter is forced into a fastretransmission. In FIGS. 4 a and 4 b this event occurs in the transitionbetween the first zone I and the second zone II, and in the transitionbetween the second zone II and the third zone III, and in the transitionbetween the third zone III and the fourth zone II.

The permitted queue latency may be increased stepwise in the transitionbetween the different zones, i.e. on or after the TCP/IP transmitter isforced into a fast retransmission, as depicted in FIG. 4 b, but may beincreased linearly in the different zones or may be increased stepwisein the zones. The increase in TCP/IP rate in FIG. 4 a is linear in thesecond, third and forth zones II-IV, but the increase in TCP/IP may bestepwise or non-linear dependent on the increase of the permitted queuelatency.

FIG. 5 a schematically shows a diagram of channel quality dependent ontime for a UE according to prior art. In FIG. 5 a it is shown that thechannel quality changes with time.

FIG. 5 b schematically shows a diagram of transmitted data from a TCP/IPtransmitter dependent on time according to prior art. In FIG. 5 a thedata segments are labelled D1-D9. The data segments D1-D9 are stored inthe queue buffer device in the Node and the scheduler has to controleach queue buffer device and the Node transmitter to transmit to thedifferent UEs in the cell. In FIG. 5 b is shown that the permitted queuelatency is set to a value corresponding to the constant value in FIG. 3b.

FIG. 5 c schematically shows a diagram of selected transmissionopportunities when the maximum permitted queue latency is shortaccording to prior art. The permitted queue latency in FIGS. 5 b and 5 cis so short that the scheduler cannot wait for a good channel qualitywhen scheduling the Node transmitter to send the buffered segments tothe dedicated UE. Hence, for example, the segments D1, D5 and D6 have tobe transmitted even though the channel quality is poor. This action useslot of transmission capacity and there is a risk for numerousretransmissions that may end up with a slow start mode.

FIG. 6 a is the same as FIG. 5 a and FIG. 6 b is the same as FIG. 5 b.However in FIG. 6 b it is shown that the permitted queue latency isincreased compared to in FIG. 5 b.

FIG. 6 c schematically shows a diagram of selected transmissionopportunities when the permitted queue latency is increased, i.e. longerthan in prior art. The scheduler may use the longer permitted queuelatency for allocating the transmission of the segments D1-D4 and D5-D9respectively when there is good channel quality. The need fortransmission capacity is thus reduced compared to prior art. Thescheduler uses the values from the rate measurement device to controlthe TCP/IP rate by use of the AQM to keep the TCP/IP oscillating aboutthe TCP/IP mean value X_2 in FIG. 4 a, i.e. oscillating between thevalue of X-max and half the X_max.

To sum up the invention the scheduler according to the invention selectsthe queue buffer device, or the queue buffer devices, that shall be usedfor transmitting data in the current scheduling interval based onchannel performance for each UE and for allowed permitted queue latencyfor each queue. However, other mechanisms may also affect the queueselection decision for the scheduler.

Furthermore, the scheduler forces the TCP/IP transmitter to a fastretransmission mode in order to allow a longer latency time for aspecific queue and thereby make it possible for the scheduler to selecta more favourable scheduling interval, e.g. when the channel conditionsfor the UE that the queue belongs to are good, for that queue within thenew longer maximum latency time.

The following description of the invention valid for FIGS. 1, 2, 4 and6.

The rate measuring device measures the momentary data rate and forwardsthe information to the scheduler that comprises AQM. As long as themomentary rate increase, the scheduler knows that the TCP/IP-transmitteruses the slow start mode. During this phase the data latency in thequeue is minimized by the scheduler (QUEUE_LATENCY=LATENCY_MIN), i.e.the queue buffer device is scheduled as often as possible in order toraise the TCP/IP rate as fast as possible. When the scheduler determinesthat the TCP-IP rate is too high due to e.g. fairness, the schedulerorders the queue to drop the latest segment. This means that thescheduler forces the TCP/IP transmitter to a fast retransmission modeand that the TCP/IP rate is reduced to half the current TCP/IP rate. Atthe same time, the scheduler increases an internal parameterQUEUE_LATENCY to QUEUE_LATENCY=QUEUE_LATENCY+DELTA_LATENCY. Thescheduler will now permit longer latency times in the buffer. Hence thescheduling gain can increase.

After the fast retransmission, the TCP/IP-transmitter will use thecongestion avoidance mode, which means that the TCP/IP rate willincrease essentially linearly. When the scheduler again decides that therate is too high, the same procedure is performed again. The schedulerorders the queue to drop the oldest segment which leads to fastretransmission and that the TCP/IP rate will be reduced to half thecurrent TCP/IP rate. The QUEUE_LATENCY will again be increased withDELTA_LATENCY that again leads to that the scheduling gain will beincreased.

This increment of QUEUE_LATENCY can continue until it reaches a maximumpermitted latency time. In such a case QUEUE_LATENCY is not incremented.

The QUEUE_LATENCY may be increased more often than each time a fastretransmission is provoked. In such a case the DELTA_LATENCY hasreasonably to be smaller. The absolute value shall be set so that it canbe ensured that the RTT estimator in the TCP/IP transmitter can followthe changes in RTT.

The RATE MEASUREMENT measure continuously the momentary TCP/IP rate. Ifthis measurement detects that the rate is under a certain minimum rate(MIN_RATE) it means that we have detected that the TCP/IP transmitterhas restarted in ‘slow start mode’. There can be several reasons why theTCP/IP transmitter restarts in slow start.

When the RATE MEASUREMENT device informs the scheduler that thetransmission has re-entered slow start-mode, the scheduler resets theQUEUE_LATENCY parameter to LATENCY_MIN. This leads to that the scheduleragain minimizes the permitted queue latency in order to raise the TCP/IPrate as fast as possible.

One benefit of the invention is that the scheduler can wait withtransmission until the channel performance to the UE is good. Suchhandling leads to increased scheduling gain since data is transmittedonly when the channel is good. However, the higher latency will lead tohigher RTT seen for the TCP/IP sender and thereby the TCP/IP rate willramp up more slowly.

The invention minimizes the round trip time, RTT, during a slowstart-phase resulting in that the TCP/IP rate increase is maximized.Then, when the TCP/IP rate have entered a congestion avoidance-phase(which is equal to that the TCP/IP rate has reached a high level), thepermitted data latency in the base station can be increased for betterscheduling gain.

Here “better scheduling gain” refers to the increased total data rate ina cell that can be achieved due to the fact that the probability to beable to send data over a good channel increases when the transmissionopportunity can be selected over a longer time, i.e. the permitted datalatency in the Node. Another prerequisite to utilize the scheduling gainis that the scheduler has several UEs to select between when it shalldecide to which UE data shall be transmitted to in a specifictransmission opportunity. Scheduling gain is achieved when at least oneof these UEs has good channel quality

One advantage of the invention is that the permitted data latency timein the queue buffer is increased smoothly which gives a round trip time,RTT, estimator in the TCP/IP transmitter that can follow the changes inRTT. This will further lead to minimized risk of a round trip timetimeout, RTO, which would have led to a slow start mode due to changesin RTT.

The RTT estimator is a function in the TCP protocol that is assigned toadapt the TCP protocol to a connection with arbitrary round trip time.The timer for round trip time out (RTO) is then set as a function of theestimated round trip time.

The invention allows the possibility to combine the desire to have lowlatency in the TCP/IP transmitter in order to have a fast rate ramp up,with the desire to have high latency in the scheduler in order toachieve high scheduling gain. High scheduling gain results in highertotal data throughput that will be a benefit for all UEs in the cell.

Abbreviations 3GPP 3rd Generation Partnership Project ACK AcknowledgmentAQM Active Queue Management IEEE Institute of Electrical and ElectronicsEngineers IP Internet Protocol OSI Open Systems Interconnection (the OSIReference Model)

RTO roundtrip time timeoutRTT roundtrip time

TCP Transmission Control Protocol UDP User Datagram Protocol

UE User Equipment, e.g. cellular telephone or wireless computer

1. A scheduler for a TCP/IP based data communication system, comprising:a TCP/IP transmitter and a receiving unit (UE), wherein the scheduler isassociated with a Node comprising a rate measuring device for measuringa TCP/IP data rate from the TCP/IP transmitter and a queue buffer devicefor buffering data segments from the TCP/IP transmitter, wherein thescheduler receives information from the rate measuring device regardingthe TCP/IP data rate, wherein the scheduler communicates with the bufferdevice for active queue management (AQM), wherein the scheduler performsthe following steps: adapt the permitted queue latency to a minimumvalue when the TCP/IP transmitter is in a slow start mode and to;increase the permitted queue latency when the TCP/IP rate has reached athreshold value.
 2. The scheduler according to claim 1 wherein thescheduler increases the permitted queue latency (QUEUE_LATENCY) until aselected maximum permitted latency time is reached.
 3. The scheduleraccording to claim 1, wherein the scheduler increases the permittedqueue latency with a rate low enough to avoid a round trip timeout, RTO.4. The scheduler according to any one of the preceding claims, whereinthe scheduler increases the permitted queue latency when, or after, thethreshold value for the TCP/IP rate corresponds to when the scheduleruses the AQM to force the TCP/IP transmitter to a fast retransmissionmode.
 5. The scheduler according to claim 4, wherein the schedulerincreases the permitted queue latency stepwise when, or after, thescheduler uses the AQM to force the TCP/IP transmitter to the fastretransmission mode.
 6. The scheduler according to claim 4, wherein thescheduler controls the queue buffer device to drop at least one datasegment, preferably the oldest, when the TCP/IP rate has reached itsthreshold value in order to force the TCP/IP transmitter to a fastretransmission mode.
 7. The scheduler according to any one of thepreceding claims, wherein the scheduler resets the permitted queuelatency to its minimum value if the measured TCP/IP rate is below aselected minimum rate in order to identify if the TCP/IP is in a slowstart mode.
 8. The scheduler according to claim 1 wherein the schedulerhandles both uplink and downlink from a UE point of view.
 9. Thescheduler according to claim 1, wherein the data communication systemcomprises an air interface between the TCP/IP transmitter and thereceiving unit.
 10. The scheduler according to claim 9, wherein thecommunication system is a wireless telephone and/or data communicationnetwork.
 11. A method for a scheduler for a TCP/IP based datacommunication system, comprising: a TCP/IP transmitter and a receivingunit (UE), wherein the scheduler is associated with a Node comprising arate measuring device for measuring a TCP/IP data rate from the TCP/IPtransmitter and a queue buffer device for buffering data segments fromthe TCP/IP transmitter, wherein the scheduler receives information fromthe rate measuring device regarding the TCP/IP data rate, wherein thescheduler communicates with the buffer device for active queuemanagement AQM, wherein the scheduler performs the following steps:adapt the permitted queue latency to a minimum value when the TCP/IPtransmitter is in a slow start mode and to; increase the permitted queuelatency when the TCP/IP rate has reached a threshold value.
 12. Themethod according to claim 11, wherein the scheduler increases thepermitted queue latency (QUEUE_LATENCY) until a selected maximumpermitted latency time is reached.
 13. The method according to claim 12,wherein the scheduler increases the permitted queue latency with a ratelow enough to avoid a round trip time out, RTO.
 14. The method accordingto claim 11, wherein the scheduler increases the permitted queue latencywhen, or after, the threshold value for the TCP/IP rate corresponds towhen the scheduler uses the AQM to force the TCP/IP transmitter to afast retransmission mode.
 15. The method according to claim 14, whereinthe scheduler increases the permitted queue latency stepwise when, orafter, the scheduler uses the AQM to force the TCP/IP transmitter to thefast retransmission mode.
 16. The method according to claim 15 whereinthe scheduler controls the queue buffer device to drop at least one datasegment, preferably the oldest, when the TCP/IP rate has reached itsthreshold value in order to force the TCP/IP transmitter to a fastretransmission mode.
 17. The method according to claim 11, wherein thescheduler resets the permitted queue latency to its minimum value if themeasured TCP/IP rate is below a selected minimum rate in order toidentify if the TCP/IP is in a slow start mode.
 18. The method accordingto claim 11, wherein the scheduler manages both uplink and downlink froma UE point of view.
 19. The method according to claim 11, wherein thedata communication system comprises an air interface between the TCP/IPtransmitter and the receiving unit.
 20. The method according to claim19, wherein the communication system is a wireless telephone and/or datacommunication network.